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arXiv:2201.04544 · Physical Review A

Rapid generation of all-optical K39 Bose-Einstein condensates using a low-field Feshbach resonance

Alexander Herbst, H. Albers, Knut Stolzenberg, Sebastian Bode, Dennis Schlippert

Published Oct 21, 2022• 8 citations
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Abstract

Ultracold potassium is an interesting candidate for quantum technology applications and fundamental research as it allows controlling intra-atomic interactions via low-field magnetic Feshbach resonances. However, the realization of high-flux sources of Bose-Einstein condensates remains challenging due to the necessity of optical trapping to use magnetic fields as free parameters. We investigate the production of all-optical $^{39}\mathrm{K}$ Bose-Einstein condensates with different scattering lengths using a Feshbach resonance near 33 G. By tuning the scattering length in a range between $75{a}_{0}$ and $300{a}_{0}$ we demonstrate a tradeoff between evaporation speed and final atom number and decrease our evaporation time by a factor of 5 while approximately doubling the evaporation flux. To this end, we are able to produce fully condensed ensembles with $5.8\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms within $850\text{\ensuremath{-}}\mathrm{ms}$ evaporation time at a scattering length of $232{a}_{0}$ and $1.6\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms within $3.9\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ at $158{a}_{0}$, respectively. We deploy a numerical model to analyze the flux and atom number scaling with respect to scattering length, identify current limitations, and simulate the optimal performance of our setup. Based on our findings we describe routes towards high-flux sources of ultracold potassium for inertial sensing.

DOI
10.1103/physreva.106.043320

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Ultracold potassium is an interesting candidate for quantum technology applications and fundamental research as it allows controlling intra-atomic interactions via low-field magnetic Feshbach resonances. However, the realization of high-flux sources of Bose-Einstein condensates remains challenging due to the necessity of optical trapping to use magnetic fields as free parameters. We investigate the production of all-optical 39K^{39}\mathrm{K}39K Bose-Einstein condensates with different scattering lengths using a Feshbach resonance near 33 G. By tuning the scattering length in a range between 75a075{a}_{0}75a0​ and 300a0300{a}_{0}300a0​ we demonstrate a tradeoff between evaporation speed and final atom number and decrease our evaporation time by a factor of 5 while approximately doubling the evaporation flux. To this end, we are able to produce fully condensed ensembles with 5.8\ifmmode×\else\texttimes\fi1045.8\ifmmode\times\else\texttimes\fi{}{10}^{4}5.8\ifmmode×\else\texttimes\fi104 atoms within 850\ensuremath-ms850\text{\ensuremath{-}}\mathrm{ms}850\ensuremath-ms evaporation time at a scattering length of 232a0232{a}_{0}232a0​ and 1.6\ifmmode×\else\texttimes\fi1051.6\ifmmode\times\else\texttimes\fi{}{10}^{5}1.6\ifmmode×\else\texttimes\fi105 atoms within 3.9s3.9\phantom{\rule{0.16em}{0ex}}\mathrm{s}3.9s at 158a0158{a}_{0}158a0​, respectively. We deploy a numerical model to analyze the flux and atom number scaling with respect to scattering length, identify current limitations, and simulate the optimal performance of our setup. Based on our findings we describe routes towards high-flux sources of ultracold potassium for inertial sensing.
We report the all-optical production of Bose-Einstein condensates (BEC) of 39K^{39}\mathrm{K}39K atoms. We directly load $3\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms in a large volume optical dipole trap from gray molasses on the D1 transition. We then apply a small magnetic quadrupole field to polarize the sample before transferring the atoms in a tightly confining optical trap. Evaporative cooling is finally performed close to a Feshbach resonance to enhance the scattering length. Our setup allows one to cross the BEC threshold with $3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms every 7 s. As an illustration of the interest of the tunability of the interactions we study the expansion of Bose-Einstein condensates in the one-dimensional to three-dimensional crossover.
Atom interferometers allow determining inertial effects to high accuracy. Quantum-projection noise as well as systematic effects impose demands on large atomic flux as well as ultralow expansion rates. Here we report on a high-flux source of ultracold atoms with free expansion rates near the Heisenberg limit directly upon release from the trap. Our results are achieved in a time-averaged optical dipole trap and enabled through dynamic tuning of the atomic scattering length across two orders of magnitude interaction strength via magnetic Feshbach resonances. We demonstrate Bose-Einstein condensates with more than 6×104 particles after evaporative cooling for 170 ms and their subsequent release with a minimal expansion energy of 4.5 nK in one direction. Based on our results we estimate the performance of an atom interferometer and compare our source system to a high performance chip trap, as readily available for ultraprecise measurements in microgravity environments. Published by the American Physical Society 2024
The hyperfine-state-selected scattering properties of potassium atoms at ultralow temperatures are calculated using interaction potentials gleaned from an analysis of recent photoassociation data. We predict that the small, probably negative value of the 39K{}^{39}\mathrm{K}39K triplet scattering length will hamper efforts to produce a Bose-Einstein condensate, unless experiments utilize a broad, accessible magnetic Feshbach resonance. The large positive value calculated for the 41K{}^{41}\mathrm{K}41K triplet scattering length makes it a better candidate for condensation at zero magnetic field. The fermionic isotope 40K{}^{40}\mathrm{K}40K is also predicted to have a large, positive scattering length for elastic collisions between spin states of experimental interest, implying that it can be efficiently evaporatively cooled to the quantum degenerate regime. In addition, certain spin states possess Feshbach resonances that may enable tuning of its interatomic interactions, possibly leading to the formation of Cooper pairs.
We describe an experimental setup and the cooling procedure for producing 39K^{39}\mathrm{K}39K Bose-Einstein condensates of over $4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms. Condensation is achieved via a combination of sympathetic cooling with 87Rb^{87}\mathrm{Rb}87Rb in a quadrupole-Ioffe-configuration (QUIC) magnetic trap and direct evaporation in a large-volume crossed optical dipole trap, where we exploit the broad Feshbach resonance at 402 G to tune the 39K^{39}\mathrm{K}39K interactions from weak and attractive to strong and repulsive. In the same apparatus we create quasipure 87Rb^{87}\mathrm{Rb}87Rb condensates of over $8\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms.
We produce a Bose-Einstein condensate of 39K atoms. Condensation of this species with a naturally small and negative scattering length is achieved by a combination of sympathetic cooling with 87Rb and direct evaporation, exploiting the magnetic tuning of both inter- and intraspecies interactions at Feshbach resonances. We explore the tunability of the self-interactions by studying the expansion and the stability of the condensate. We find that a 39K condensate is interesting for future experiments requiring a weakly-interacting Bose gas.
Bose-Einstein condensation has been achieved in a magnetically trapped sample of 85Rb atoms. Long-lived condensates of up to 10(4) atoms have been produced by using a magnetic-field-induced Feshbach resonance to reverse the sign of the scattering length. This system provides new opportunities for the study of condensate physics. The variation of the scattering length near the resonance has been used to magnetically tune the condensate self-interaction energy over a wide range, extending from strong repulsive to large attractive interactions. When the interactions were switched from repulsive to attractive, the condensate shrank to below our resolution limit, and after approximately 5 ms emitted a burst of high-energy atoms.